1. Field of the Invention
The invention relates to a display device using phosphors to display the dots of an image. The invention is more particularly applicable to plasma display panels and to cathode-ray tubes using high scanning frequencies.
2. Description of the Prior Art
Plasma display panels (PDP) and cathode-ray tubes (CRT) comprise, on their front face, a layer made of a luminescent material which converts either UV radiation or electron radiation into visible light radiation. The luminescent material is commonly called a phosphor.
For monochromatic screens, the same phosphor is used over the entire front face of the CRT or PDP. On the other hand, for colour screens, three types of phosphor having different colours are generally used in order to synthesize colour. For specific applications, it is possible to have screens using two or more than three types of phosphor.
The use of phosphors having different colours exhibits some operational disparities due to the intrinsic characteristics of the materials forming the phosphors. Among the operational disparities, the temporal response to excitation is specific to each type of phosphor.
For CRTs, this fault is not generally perceived on low-definition screens, for example of TV type. However, it is possible to perceive slight faults in very-high-definition screens (for example 1600×1200 pixels) using high refresh frequencies (for example >120 Hz).
For PDPs, the disparities are very large. FIG. 1 shows phosphor reaction time diagrams commonly used in PDPs. FIG. 1A shows an excitation time period during which electrical discharges are sent into the panel in order to produce UV radiation (not shown). The UV radiation is then converted into visible light by the phosphors. FIG. 1B shows the light rendition for a blue phosphor, for example a barium magnesium aluminate doped with divalent europium. FIG. 1C shows the light rendition for a red phosphor, for example an yttrium borate doped with trivalent europium. FIG. 1D shows the light rendition for a green phosphor, for example a barium aluminate doped with manganese.
FIGS. 1B to 1D have different vertical scales which make the maximum values of each of the curves correspond. In reality, the maximum blue value is about 4.3 times greater than the maximum red value and about 5.5 times greater than the maximum green value. However, the light energy efficiency is substantially the same for each of the colours. These time diagrams make it possible to display the energy distribution per colour. By way of example, for a given excitation, the time durations for which the emitted light becomes less than 10% of the maximum emission value is indicated. Thus, less than one millisecond after the end of excitation, the blue colour is virtually extinguished while the red and green colours are still close to their maximum level, extinction of the red and of the green corresponding respectively to 11 and 13 ms.
FIG. 1E shows, on the one hand, the light renditions of the three colours with the same light intensity scale and on the other hand the sum of the three light renditions which corresponds to a pixel seen by the human eye. If the colour corresponding to the sum of the three renditions is looked at, it is noticed that the pixel is initially blue, then passes from blue to white (or grey depending on the intensity), then passes from white to yellow (combination of green and red of substantially the same intensity), and finally passes from yellow to green before being extinguished. In PDPs, the discharges repeat cyclically at the screen refresh frequency.
In the case of a stationary image, the persistence of vision of the human eye carries out filtering of low-pass type on the colour variations which masks this defect.
On the other hand, with a moving image, the eye becomes more sensitive to the colour variation at the colour transitions, which are displaced. Thus, a white object moving on a black background for example takes on a blue leading edge and a yellow trailing edge (green is not perceptible by the human eye in our example).
To overcome this type of problem, the only known solutions are to find novel phosphors in order to be able to use three types of phosphor having similar properties.